Patent GB-1469213-A (GB)
Title: Ac elevator control system
IPC: H02P5/747
Filed: 1975-03-05
Abstract: 1469213 A.C. motor control systems HITACHI Ltd 5 March 1975 [8 March. 1974] 9168/75 Heading H2J In an elevator control system, a multispeed induction motor unit, which may be two motors having different synchronous speeds or a single motor having two windings or a single motor of the one-winding two speed type, is connected to a. three-phase A.C. power supply, the elevator car being started, accelerated and driven normally by the higher speed motor and being decelerated by a regenerative braking torque produced in the lower speed motor until an instant before this braking torque becomes ineffective, at which instant a further braking torque is initiated and is controlled until the elevator car comes to rest. As shown in Fig. 1, a high speed.motor 1 and a low speed motor 2 are provided, the contactors 9 and 11 being closed to drive the motor 1 under control of a speed feed back system 14, 15, 161 and 10 during starting, accelerating and normal running. When the car is to be stopped, the contactors 11 are opened and the contactors 12 are closed so that the motor 2 is energized and produces the regenerative braking torque under control of the speed feedback system. An instant before the regenerative braking torque becomes ineffective is detected by a switching point detector 17, Fig. 5 (not shown), responsive to the motor speed; at which instant the phase shifter 16I is switched out and a phase shifter 162 is switched in to control a rectifier 13 connected to motor 1 so that a D.C. braking torque is provided in motor 1 under control of the rectifier. The phase shifters are described with reference to Fig. 3 (not shown), and include means to ensure that the A.C. voltage applied to the low speed motor is gradually reduced-after the shifter 161 is switched out, and that the D.C. voltage applied to the motor 1 is gradually increased when the shifter 162 is switched in so that a smooth transition between the application of the regenerative braking torque and the D.C. braking torque is achieved. A modification is described with reference to Fig. 10 (not shown) in which the rectifier 13 is connected to an electromagnetic braking means. An alternative embodiment is described with reference to Fig. 11 (not shown) in which the motors 1 and 2 are replaced by a one-winding two-speed motor, and this embodiment is modified as shown in Fig. 12. In Fig. 12, the rectifier 13 is omitted, and when the regenerative braking torque becomes ineffective, a D.C. braking torque is provided by way of thyristors 101, 103, being turned on, free-wheel thyristors being provided. In a further modification, a one-winding two speed motor may be employed as the-low speed induction motor whilst retaining the high speed induction motor 1. In an elevator system in which the car is controlled by regulating the braking torque by means of a load detector, the output of the load detector, e.g. a differential transformer, is applied to a memory, Fig. 13 (not shown). A braking torque instruction signal generator, Fig. 14 (not shown) generates a voltage-which is variable step-wise depending on the contents of the memory, and this instruction signal is applied to the phase shifters 161 and 162, and the tachometer 3, generator 14 and comparator 15 may then be dispensed with. Another form of switching point detector 17 is described with reference to Fig. 16 (not shown), this detector being in the form of a differential amplifier responsive to elevatorcar speed and load voltages. The load voltage may be made dependent upon the moving direction of the elevator car by means of the circuit described with reference to Fig. 17 (not shown).